MEMS Device

ABSTRACT

System and method for a microelectromechanical system (MEMS) is disclosed. A preferred embodiment comprises a first anchor region, a vibrating MEMS structure fixed to the first anchor region, a first electrode adjacent the vibrating MEMS structure, a second electrode adjacent the vibrating MEMS structure wherein the vibrating MEMS structure is arranged between the first and the second electrode.

This application is a continuation of U.S. patent application Ser. No.12/474,368, entitled “MEMS Device,” filed on May 29, 2009 and is herebyincorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to a system and method ofresonator devices, and more particularly to a system and method formicroelectromechanical system (MEMS) resonator devices.

BACKGROUND

A resonator is a device that exhibits resonant behavior, that is, itoscillates at some frequencies with greater amplitude than at otherfrequencies. A resonator usually oscillates at specific frequenciesbecause its properties and dimensions are an integral multiple of thewavelength at those frequencies. Resonators may be used to generatewaves of specific frequencies or to select specific frequencies from asignal.

In some applications it is desirable to replace a quartz crystal with amicroelectromechanical system (MEMS) resonator. For example, effortshave been made to introduce radio frequency (RF) MEMS devices for timingapplications. Compared with quartz crystals, MEMS resonators can providereduced size as well as improved integration with an oscillator orapplication specific integrated circuits (ASIC), thereby providingreduced overall system costs.

To meet application specifications, a MEMS resonator device often needsto have several characteristics at the same time. These characteristicscan include high frequency stability, low supply voltage, low impedancesupporting low power consumption, low phase noise and fast start upbehavior. To achieve high compatibility for different applications, itis desired to have a variable resonator frequency that is scalable bydesign rather than by process change. The performance parameters of theresonators depend on the process concept, such as materials, processstability and to a large extent on the resonator design itself.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, amicroelectromechanical system (MEMS) is disclosed. The MEMS includes afirst anchor region, a vibrating MEMS structure fixed to the firstanchor region, a first electrode adjacent the vibrating MEMS structureand a second electrode adjacent the vibrating MEMS structure. Thevibrating MEMS structure is arranged between the first and the secondelectrode.

In accordance with another preferred embodiment of the presentinvention, a method for adjusting a resonance frequency of amicroelectromechanical system (MEMS) is disclosed. The method includesproviding a MEMS resonator comprising a resonator element, a firstelectrode and a second electrode, the resonator element being arrangedbetween the first and the second electrode. The method further includesapplying a first bias voltage to the resonator element and the firstelectrode and applying a second bias voltage to the second electrode,wherein the second bias voltage is independent from the first biasvoltage.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 is an embodiment of a MEMS resonator with two anchor regions;

FIG. 2 is another embodiment of a MEMS resonator with two anchorregions;

FIG. 3 is a top view of an embodiment of a MEMS resonator;

FIG. 4 is an embodiment of a MEMS resonator with one anchor region.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, namely in MEMS resonator devices. Theinvention may also be applied to yet other embodiments such as resonatordevices. Other applications are also foreseen.

Accuracy is one of the major topics in MEMS technology. Small processdeviations (e.g. lithography) can lead to a slightly changed mechanicalbehavior. For resonator applications of MEMS devices, the resonancefrequency (f) is determined by the moving mass (m) and the stiffness (k)of its resonator:

$f = {\frac{1}{2\pi}\sqrt{\frac{k}{m}}}$

Small process variations can lead to either a slightly changed mass or aslightly changed stiffness or both. Therefore, the resonance frequency(f) will slightly change too:

$f = {\frac{1}{2\pi}\sqrt{\frac{k + {\Delta \; k}}{m + {\Delta \; m}}}}$

Conventional methods provide active or passive trimming techniques toadjust the resonance frequency (f) of a resonator device.

With passive trimming techniques the resonator devices are trimmeddirectly after wafer level test. Two techniques have been proposed: Adeposition technique and a laser trimming technique. Both change themass of the resonator and therefore the resonance frequency. Asignificant disadvantage of the deposition technique is that it can onlybe used with devices which are not sealed, i.e. where the device itselfis directly accessible. In contrast, laser trimming is applicable tosealed resonators. However, laser trimming techniques require multipletest and run cycles which make production costly.

The most commonly used trimming techniques are active trimmingtechniques. One active trimming technique uses a phase locked loop(PLL). A PLL is an active system and contributes to noise which in turnhas a negative influence on the noise performance of the resonatorsystem.

Another active trimming technique is heating the resonator device.However, heating the device to alter the material properties requiressignificant power and is therefore not desirable.

Still, a further active trimming technique is bias voltage trimmingwhich makes use of an effect called electrical spring softening.Electrical spring softening leads to a shift in resonance frequency byapplying a bias voltage (V_(Bias)). The electrical spring softening(k_(elec)) depends on a bias voltage (V_(Bias)) applied between anelectrode and a resonator. The electrical spring softening (k_(elec)) isfurther influenced by a gap distance (d) and an area (A) between both ofthem.

$k_{elec} = {{- \frac{ɛ\; A}{d^{3}}}V_{Bias}}$

The influence of the electrical spring softening on the resonancefrequency (f) can be described as follows:

$f = {\frac{1}{2\pi}\sqrt{\frac{k + k_{elec}}{m}}}$

Performance parameters such as quality factor and motional resistancestrongly depend on electro-mechanical coupling (η). Theelectro-mechanical coupling factor (η) itself depends from a capacitance(C) between the electrode and the resonator, a gap (d) between both andan applied bias voltage (V_(Bias)).

$\eta = {\frac{C}{d}V_{Bias}}$

Large values of the bias voltage (V_(Bias)) may substantially influencethe other performance parameters. In order to avoid such a substantialimpact on these parameters, the use of the bias voltage (V_(Bias)) toadjust the resonance frequency (f) may be limited to a trimming range ofsmall values of several tens to some hundreds of parts per million(ppm). However, the electrical and quality performance of the resonatordevice may be insufficient with decreasing bias voltage (V_(Bias)), forexample.

Further, as the resonance frequency (f) is bias voltage (V_(Bias))dependent any amplitude noise of the bias supply may directly translateto phase noise within the resonator of the MEMS device. This may have asubstantial impact on the output frequency of high level RF MEMSapplications such as GSM, UMTS or WCDMA.

To provide more freedom in adjusting the resonance frequency (f) anadditional electrical field is applied to the resonator device. Theadditional electrical field is introduced by an additional electrode,for example. Alternatively, the additional electrical field or theadditional electrical fields may be introduced by a plurality ofadditional electrodes. The additional electrical field(s) or theadditional electrode(s) will provide an additional degree of freedom inadjusting the resonator devices and more particular the resonancefrequencies of MEMS resonators, for example. The additional electricalfield(s) or the additional electrode(s) may only influence otherperformance parameters in a very limited way in other embodiments.

Therefore, an additional electrical stiffness (k_(trim)) can be added tothe resonance frequency equation and the equation can be written as

$f = {\frac{1}{2\pi}\sqrt{\frac{k + k_{elec} + k_{trim}}{m}}}$

Finally, embodiments of the inventive method may guarantee frequencytuning throughout the lifetime of the MEMS device.

Referring now to FIG. 1, a schematic view of a first embodiment of aMEMS device 100 is shown. The MEMS device 100 includes a vibratingstructure or resonator element, typically a resonator electrode 101, afirst electrode or drive electrode 102 and a second electrode orexternal electrode 103. In one embodiment, the MEMS device 100 is aclamped-clamped beam resonator (CC Beam) wherein the beam or theresonator electrode 101 is anchored to the substrate at the top 104 andthe bottom 105.

The MEMS device 100 may be made of silicon wherein the moving part, i.e.the resonator electrode 101, may comprise polysilicon. Alternatively,the MEMS device 100 includes a mono-crystalline silicon layer such as asilicon on insulator (SOI) substrate. Such a resonator device 100benefits from well-defined mono-crystalline material properties. MEMSdevice 100 may be based on materials other than pure silicon, forexample, silicon germanium (SiGe).

Different designs can address different performance parameters. Forexample, vibrating structures or resonator elements such as beamstructures operating in a flexural mode have a relatively smallmechanical spring constant and can therefore achieve a low impedance ata low supply voltage. Other designs may operate in pure breath mode orbulk acoustic mode and may have larger mechanical spring constantscompared to the flexural beam design.

One way of optimizing resonator device parameters, such as good phasenoise, low impedance and low bias voltage, is to start with a bulkacoustic mode design and optimize it for low bias voltage and lowimpedance. Resonator gap width, mechanical spring constant, andresonator area can be varied to achieve these results.

In one embodiment a desired resonance frequency (f) may be set byapplying a DC bias voltage (V_(DC)) across the resonator electrode 101and the drive electrode 102. V_(DC) is also responsible forelectro-mechanical coupling (η) and, therefore, has an effect on otherimportant parameters such as quality factor and motional resistance.

To further tune the frequency towards the desired resonance frequency(f) an additional trimming voltage (V_(trim)) may be applied to theexternal electrode 103. The DC potential difference between theresonator electrode 101 and the external electrode 103 isV_(trim)-V_(DC). By varying only V_(trim) the potential differencebetween the external electrode 103 and the resonator electrode 101 mayvary, whereas the potential difference between the drive electrode 102and the resonator electrode 101 (V_(DC)) may remain constant.

Since the potential difference between the resonator electrode 101 andthe drive electrode 102 may remain virtually uninfluenced by thevariation of the potential of the external electrode 103, theelectro-mechanical coupling factor (η) may also remain virtuallyuninfluenced by the potential variation of the external electrode 103.

Trimming the resonance frequency (f) by varying the potential of anexternal electrode 103 may have only a very limited adverse effect onthe other performance parameters. A trimming in a range of a few tens ofparts per million (ppm) may be possible with a very insignificant effecton the other performance parameters and trimming in a larger range mayhave only a limited effect on these parameters.

When V_(trim) is either greater than V_(DC) or less than V_(DC) anattractive force F_(ext) 110 is generated on the resonator electrode 101pulling the resonator electrode 101 towards the external electrode 103.The attractive force F_(ext) 110 reduces the effect of the electricalforce F_(el) 112, the force between the resonator electrode 101 and thedrive electrode 102. The difference in the potentials V_(trim)-V_(DC)has therefore an effect on the capacitance C_(e) 120, between theresonator electrode 101 and the external electrode 103, and thecapacitance C_(o) 122 between the resonator electrode 101 and the driveelectrode 102. For example, the attractive force F_(ext) 110 mayincrease the capacitance C_(e) 120 and, at the same time, may reduce thecapacitance C_(o) 122.

The MEMS device 100 may be operated by applying an AC voltage signal(V_(AC)) at the drive electrode 102 and by sensing it at the resonatorelectrode 101. When the frequency of the AC voltage signal matches theresonance frequency of the resonator electrode 101 the impedance of thepath is reduced and hence a larger signal is sensed at the resonatorelectrode 101.

In one embodiment, the second order temperature coefficient of theresonance frequency may be compensated by the proposed trimmingtechnique, especially by applying a trimming voltage (V_(trim)) via theexternal electrode 103. In one embodiment, the application of theproposed trimming technique may be combined with the use of an oxidefilling technique which is described in a related patent applicationSer. No. 12/187,443 “Passive Temperature Compensation of Silicon MEMSDevices” and which is herein incorporated by reference. The oxidefilling technique may compensate the first order temperature coefficientof the resonance frequency while the proposed voltage trimming techniquemay compensate the second order temperature coefficient of the resonancefrequency.

The combination of both techniques may reduce the drift resonancefrequency in an exemplary temperature range from −10 C. to 95 C. fromabout 2000 ppm (0.2% drift) to 0.8 ppm (8e-5%) which makes it suitablefor GSM applications, for example.

The trimming voltage (V_(trim)) may further depend on a temperature. Inone embodiment the dependence on the temperature is quite identical tothat of a bandgap voltage reference source of about 1.25 V. Using thebandgap reference voltage as source makes the embodiments of the presentinventions desirable for noise sensitive applications such as GSM

FIG. 2 shows a cross sectional view of an embodiment of a MEMS devise inform of a plate MEMS resonator device. MEMS resonator device 200 is anexample of CC Beam resonator. FIG. 2 shows a CC Beam or first movableplate electrode 201, a second electrode or drive electrode 202 and athird electrode or external electrode 203. The movable plate electrode201 is fixed via the anchor regions 205, 207 to the substrate 220. Thesecond electrode or drive electrode is 202 is placed atop of thesubstrate 220 and the third electrode or external electrode 203 is fixedvia the anchor regions 205-211 to the substrate 220.

Applying a DC bias voltage (V_(DC)) over he first movable plateelectrode 201 and the drive electrode 202 attracts the first movableplate electrode 201 toward the drive electrode 202 resulting in anelectrostatic force F_(el) 212. The electrostatic force F_(el) 212 movesthe first movable plate electrode 201 towards the drive electrode 202until an equilibrium between the spring force and the electrostaticforce F_(el) 212 is reached resulting in a spring constant (k_(elec)).

Applying a positive or negative trimming voltage (V_(trim)) to theexternal electrode 203 moves the first movable plate electrode 201towards the external electrode 203 resulting in an attractive forceF_(ext) 210 and therefore in an additional spring constant (k_(trim))reducing the effect of the electrostatic force F_(el) 212

FIG. 3 shows another embodiment of a MEMS devise in form of a wheelshaped MEMS resonator device. In one embodiment, device 300 comprises awheel mass 301 which functions as resonator electrode. The wheel mass301 is spaced apart from the drive electrode 302 by a gap 306. Wheelmass 301 is round in one embodiment, although other shapes andconfigurations are possible. For example, resonator may have acircumference, a periphery with at least a portion having a radius, orsome other shape in embodiments.

Drive electrode 302 and gap 306 extend along a circumference of MEMSresonator device 300. The drive electrode 302 may comprise a singleelectrode but, alternatively, may comprise a plurality of electrodes.MEMS resonator device 300 also comprises an anchor region 307 coupledthe wheel mass 302 by a plurality of beam elements 308. While resonatordevice 300 comprises eight beam elements 308, the number, placement andconfiguration can vary in other embodiments. A plurality of apertures309 are formed in the anchor region 307 and anchor region 307 is coupledto a substrate by an anchor portion 304. The number, configuration andplacement of apertures 309 can vary in other embodiments. MEMS resonatordevice 300 also comprises one or more external electrodes 303.

Drive electrode 302 provides a large electrode area, and wheel mass 301is configured to resonate or vibrate in a radial breath, or longitudinalmode capable of a large mechanical spring constant.

External electrodes 303 provide also a large electrode area to tunewheel mass 301 efficiently. The external electrodes 303 can beelectrically contacted individually or in any configuration, e.g.grouping the external electrodes 303 into 2, 4 or 8 elements.

Applying a bias voltage (V_(Bias)) over the wheel mass 301 and the driveelectrode 302 attracts the wheel mass 301 toward the drive electrode 302resulting in an electrostatic force F_(el). The electrostatic forceF_(el) moves the wheel mass 301 towards the drive electrode 302 until afirst equilibrium between the spring force and the electrostatic forceF_(el) is reached resulting in a spring constant (k_(elec)).

Applying a positive trimming voltage (V_(trim)) to the one or moreexternal electrodes 303 moves the wheel mass 301 towards the externalelectrode 303 resulting in an additional spring constant (k_(trim))reducing the effect of the electrostatic force F_(el).

FIG. 4 illustrates a schematic view of a further embodiment of a MEMSresonator device. FIG. 4 shows a MEMS device 400 with only one anchorregion, e.g. only one part of the vibrating structure is fixed. The samenumerals will be used in this embodiment for the same or similarelements of embodiment in FIG. 1.

The MEMS resonator device 400 includes a vibrating structure orresonator element, typically a resonator electrode 401, a firstelectrode or drive electrode 402 and a second electrode externalelectrode 403. In one embodiment, the resonator electrode 401 isanchored to the substrate at the bottom 405 having one free end.

A desired resonance frequency (f) may be set in a similar way asdescribed for the CC Beam arrangement of FIG. 1. In order to set adesired resonance frequency (f) a DC bias voltage (V_(DC)) across theresonator electrode 401 and the drive electrode 402 may be set. Tofurther tune the frequency towards the desired resonance frequency (f)an additional trimming voltage (V_(trim)) may be applied to the externalelectrode 403. The DC potential difference between the resonatorelectrode 401 and the external electrode 403 is V_(trim)-V_(DC). Byvarying only V_(trim) the potential difference between the externalelectrode 403 and the resonator electrode 401 may vary, whereas thepotential difference between the drive electrode 402 and the resonatorelectrode 401 (V_(DC)) may remain constant.

When V_(trim) is either greater than V_(DC) or less than V_(DC) anattractive force F_(ext) 410 is generated on the resonator electrode 401pulling the resonator electrode 401 towards the external electrode 403.The attractive force F_(ext) 410 reduces the effect of the electricalforce F_(el) 412, the force between the resonator electrode 401 and thedrive electrode 402. The difference in the potentials V_(trim)-V_(DC)has therefore an effect on the capacitance C_(e) 420, between theresonator electrode 401 and the external electrode 403, and thecapacitance C_(o) 422 between the resonator electrode 401 and the driveelectrode 402.

The MEMS device 400 may be operated by applying an AC voltage signal(V_(AC)) at the drive electrode 402 and by sensing it at the resonatorelectrode 401. When the frequency of the AC voltage signal matches theresonance frequency of the resonator electrode 401 the impedance of thepath is reduced and hence a larger signal is sensed at the resonatorelectrode 401.

However, unlike the CC Beam arrangement in FIG. 1, the single anchoredMEMS resonator is highly non-linear and generally provides a lowresonance frequency, for example.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. For example,many of the features and functions discussed above can be implemented inother hardware solutions.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A microelectromechanical system (MEMS) comprising: a first anchorregion; a MEMS structure fixed to the first anchor region; a first driveor trim electrode located adjacent a first surface of the MEMSstructure; and a second drive or trim electrode located adjacent asecond surface of the MEMS structure.
 2. The MEMS according to claim 1,further comprising a second anchor region, wherein the MEMS structure isalso fixed to the second anchor region.
 3. The MEMS according to claim1, wherein the first drive or trim electrode comprises a plurality offirst drive or trim electrodes.
 4. The MEMS according to claim 1,wherein the second drive or trim electrode comprises a plurality ofsecond drive or trim electrodes.
 5. The MEMS according to claim 1,wherein the first drive or trim electrode comprises a plurality of firstdrive or trim electrodes, and wherein the second drive or trim electrodecomprises a plurality of second drive or trim electrodes.
 6. The MEMSaccording to claim 1, wherein the MEMS structure comprises a vibratingMEMS structure.
 7. The MEMS according to claim 1, wherein the firstsurface and the second surface are located opposite each other.
 8. TheMEMS according to claim 1, wherein the MEMS structure comprises a wheelelectrode.
 9. The MEMS according to claim 1, wherein the MEMS structurecomprises a CC beam electrode.
 10. A method for operating amicroelectromechanical system (MEMS), the method comprising: applying afirst bias voltage to a membrane of the MEMS and a first electrode,wherein the first electrode is located adjacent a first surface of themembrane, and a second electrode is located adjacent a second surface ofthe membrane; and applying a second bias voltage to the secondelectrode, wherein the second bias voltage is independent from the firstbias voltage.
 11. The method according to claim 10, wherein the firstelectrode comprises a plurality of first electrodes, and wherein thesecond electrode comprises a plurality of second electrodes.
 12. Themethod according to claim 10, wherein applying the first bias voltageshifts a stiffness of the membrane from a first stiffness to a secondstiffness.
 13. The method according to claim 12, wherein applying thesecond bias voltage shifts the second stiffness to a third stiffness.14. A method for operating a microelectromechanical system (MEMS), themethod comprising: applying a first bias voltage to a membrane of theMEMS and a first electrode, wherein the membrane is arranged between thefirst electrode and a second electrode; and applying a second biasvoltage to the second electrode, wherein the second bias voltage isindependent from the first bias voltage, wherein applying the first biasvoltage shifts a stiffness of the membrane from a first stiffness to asecond stiffness, and wherein applying the second bias voltage shiftsthe stiffness from the second stiffness to a third stiffness.
 15. Themethod according to claim 14, wherein the first electrode comprises aplurality of first electrodes.
 16. The method according to claim 14,wherein the second electrode comprises a plurality of second electrodes.17. A microelectromechanical system (MEMS) comprising: a first anchorregion; a second anchor region; a plate electrode connected to the firstanchor region and the second anchor region; a first electrode, the firstelectrode located adjacent a first surface of the plate electrode; and asecond electrode, the second electrode located adjacent a second surfaceof the plate electrode.
 18. The MEMS according to claim 17, wherein thefirst electrode comprises a plurality of first electrodes.
 19. The MEMSaccording to claim 17, wherein the second electrode comprises aplurality of second electrodes.
 20. The MEMS according to claim 17,wherein the plate electrode comprises a CC beam resonator.